Several methods and technologies have been developed for measuring the contents of large silos or bins used to store bulk materials such as minerals, coal, plastics, chemicals and the like. One approach is to determine the material level in the vessel through the use of mechanical devices or through ultrasonic sounding. Another method for measuring the contents of a large silo involves weighing the contents of the vessel. These methods become difficult and impractical, however, when the contents are stored in large, tall silos or other similarly large vessels.
Recently, bolt-on weighing has become a popular approach for measuring the contents of a vessel. This method consists of bolting strain sensors to the support structure of the vessel. The contents of a vessel can be determined from strain measurements taken from strain sensors coupled to the support structure of the vessel. By measuring the amount of axial strain in the support structure as material is added to or removed from the vessel, the weight of the material in the vessel can be determined. The use of strain sensors to weigh the contents of a vessel has several advantages over the level-sensing method. The strain sensors are positioned on the outer surface of the vessel support structure rather than inside of the vessel where the contents of the vessel may interfere with or damage the sensors. In addition, use of strain sensors requires no instruments or cabling inside the vessel, and the strain sensors can easily be installed and the service at ground level.
Strain sensors used in bolt-on weighing systems must be highly sensitive to changes in strain of the load structure, yet insensitive to temperature fluctuations. An increase in temperature of the support structure will cause the support structure to expand. Expansion of the support structure resulting from a change in temperature is unrelated to the change in weight of the material inside the vessel and thus should not be detected by the strain sensor. Therefore, strain sensors are generally made of the same material as the support structure to minimize any uneven expansion between the structure and the sensor. One of the bolt-on weighing systems commonly used today is shown in U.S. Pat. No. 4,064,744.
Some severe problems have developed, however, in utilizing bolt-on weighing. It has been found that strain sensors used in bolt-on weighing can be adversely affected by factors external to the sensor or the support structure. For example, although the sensors themselves are carefully temperature-compensated, rapid temperature changes that occur between night and day or between sunny and cloudy conditions can produce large, temporary shifts in the weight readings. These shifts occur because the thermal mass of sensor differs from the thermal mass of the structure; therefore, a change in external temperature may change the temperature of the strain sensor at a different rate than the corresponding change in temperature of the support structure. Accordingly, a temperature differential may develop between the structure and the sensor, simulating a strain which the sensor detects and includes on the sensor readout.
Another problem in obtaining accurate strain measurements through bolt-on weighing is that the sensor must be designed so that it can easily be imbedded inside the structure and located at a "nodal point" in the stress pattern, at the place of stress reversal as explained in Kistler & Nachtigal, "Errors Encountered When Using Leg-Mounted Strain Sensors to Measure Vessel Contents" in the proceedings of the Powder & Bulk Solids Conference, May 1989.
Yet another problem encountered in bolt-on weighing systems is that the sensor must be inexpensive to produce since many sensors are required to obtain accurate weight measurements.